Trace metals are important, both as essential constituents and as poisons of living organisms. Many metal ions such as Zinc and Cadmium possess few spectroscopic properties with which to probe their structural and functional role in proteins. We propose to further develop the use of the vanadyl ion (VO2 ion) as an electron paramagnetic resonance (EPR) probe capable of providing detailed information concerning metal ion-protein interactions. Procedures for introducing the vanadyl ion into proteins have been developed in this laboratory; however, the interpretation of spectral data is not always straightforward. Spectra of a variety of model compounds will be obtained at Q-band and X-band frequencies to aid in the interpretation of results with proteins. Methods for determining the number of coordinated water molecules, bound to the vanadyl probe, the rotational correlation time of the metal site, and the identity of the protein ligands will be investigated. Specifically, metal binding to transferrin, conalbumin, and leucine aminopeptidase will be examined in detail. Other proteins already labeled with vanadyl will be reinvestigated in light of the development of the new methodology mentioned above. It is intended that these studies will illustrate the wide range of applicability of the proposed vanadyl labeling method, provide needed baseline data for future work, and encourage the general use of this relatively unexplored approach to the study of metal binding in biopolymers.